As the demand for energy resources increases simultaneously with the decrease in available fossil fuels, various alternative sources of energy such as coal-based energy technologies, for example, are being deployed. Large amounts of CO2 are generated by coal gasification processes in power plants, for example, which trigger a need for carbon capture and sequestration (CCS) as well as CO2 monitoring over large areas to avoid ecologic disasters. The potential toxic gas emissions from such power plants have resulted in the need for CO2 sensors capable of being fabricated in large volumes, while offering low cost/drift/electrical power consumption and a high sensitivity and selectivity when utilized to monitor CO2 concentration in air. Additionally, such sensors in association with a wireless sensing network must be able to detect any deviation from the normal CO2 environmental concentration, which is in the range of 380-440 ppm, depending on the specific region and season.
The majority of prior art CO2 sensors are based on electrochemical and optical principles. Unfortunately, the cost for fabricating such sensors for large area CO2 monitoring is high. One of the possible sensing solutions for CO2 detection at low cost, low power consumption with high sensitivity and selectivity at low gas concentrations is the use of a resonant nano-electro-mechanical system (NEMS) sensor, which may also be referred to as a nanosensor.
The resonance frequency with respect to such a resonant nanosensor may be changed by selective CO2 adsorption and reaction on a functionalized surface of a silicon beam, which mechanically vibrates at a frequency equal to the frequency of an excitation force. Problems associated with prior art resonant sensors, and which may propagate to emerging resonant nanosensor devices, include a lack of long term performance stability and a poor drift behavior due to poor baseline stability (i.e., recovery of the sensor signal to the same response level in the absence of the gas to be detected). Other problems include its inherent temperature variations and temperature dependence of the resonance frequency, the fatigue of the vibrating beam, humidity absorption, and aging of its sensing layer, which may exhibit or contribute to a baseline drift.
Based on the foregoing, it is believed that a need exists for an improved differential resonant nanosensor apparatus for detecting carbon dioxide. A need also exists for an improved method for forming functionalized monolayers on a vibrating nanobeam surface, as described in greater detail herein.